I put the mushrooms on to fry very gently in butter, gills-down and stalk-removuff".ed. At the same time, with a little more butter, about a half onion and a good amount of garlic, both well chopped, to fry somewhat more savagely.
For the "stuffing", which it really isn't, 'cos Portabellas are too wide open to "stuff", take about a half of a pound of Jimmy Dean or Bob Evans sausage (or, if you're in Europe, minced pork), add about two teaspoonfuls of Herbes de Provence and some pepper. Mash out the meat with a fork, and then add in three eggs and keep mashing until well mixed. Now the mixture is made, start adding breadcrumbs until it all starts binding together. You need to get it to a consistency that is a little wetter than you would normally use for burgers.
Turn over the mushrooms!
By the time you're done with all this, the onion/garlic mixture should be ready to add to the meat mix, so do so.
Let the mushrooms fry gently until they start getting softer (not mushy!!), and then, for each one, take some meat mix, put it into a small pan flatten it down to about 3/10th of an inch thick and fry it, flipping once or twice. When done, sprinkle Parmesan cheese generously onto the mushroom, pop the cooked meat onto the top, and serve.